1. Technical Field
This invention relates to an organic electroluminescent (EL) device and more particularly, to an inorganic/organic junction structure suitable for use in a device of the type wherein an electric field is applied to a thin film of an organic compound to emit light.
2. Background Art
In general, organic EL devices have a basic configuration including a glass substrate, a transparent electrode of ITO etc., a hole transporting layer of an organic amine compound, a light emitting layer of an organic material exhibiting electronic conductivity and intense light emission such as Alq3, and an electrode of a metal having a low work function such as MgAg, wherein the layers are stacked on the substrate.
The device configurations which have been reported thus far have one or more organic compound layers interposed between a hole injecting electrode and an electron injecting electrode. Structures having two or three organic compound layers are typical.
Included in the two-layer structure are a structure having a hole transporting layer and a light emitting layer formed between the hole injecting electrode and the electron injecting electrode and another structure having a light emitting layer and an electron transporting layer formed between the hole injecting electrode and the electron injecting electrode. Included in the three-layer structure is a structure having a hole transporting layer, a light emitting layer, and an electron transporting layer formed between the hole injecting electrode and the electron injecting electrode. Also known is a one-layer structure wherein a single layer playing all the roles is formed from a polymer or a mixed system.
FIGS. 3 and 4 illustrate typical configurations of organic EL devices.
In FIG. 3, a hole transporting layer 14 and a light emitting layer 15, both of organic compounds, are formed between a hole injecting electrode 12 and an electron injecting electrode 13 on a substrate 11. In this configuration, the light emitting layer 15 also serves as an electron transporting layer.
In FIG. 4, a hole transporting layer 14, a light emitting layer 15, and an electron transporting layer 16, all of organic compounds, are formed between a hole injecting electrode 12 and an electron injecting electrode 13 on a substrate 11.
Reliability is a common problem to be solved for these organic EL devices. More particularly, organic EL devices in principle have a hole injecting electrode and an electron injecting electrode and need organic layers for effectively injecting and transporting holes and electrons from the electrodes, respectively. However, the organic materials of which the organic layers are formed are vulnerable during manufacture and have less affinity to the electrodes. Another problem is raised by the significantly accelerated degradation of organic thin films as compared with LED and LD.
Also, most organic materials are relatively expensive. Any partial replacement of constituent films by an inexpensive inorganic material would give an economical merit in the manufacture of cost effective organic EL device-applied products.
There is also a desire to have an organic EL device having a further improved light emission efficiency, a low drive voltage and a minimal current consumption.
To solve this and other problems, a means for taking advantage of both an organic material and an inorganic semiconductor material has been devised. That is, an organic/inorganic semiconductor junction is established by substituting an inorganic p-type semiconductor for the organic hole transporting layer. These efforts are found in Japanese Patent No. 2636341, JP-A 139893/1990, 207488/1990, and 119973/1994. However, it was impossible to design EL devices or cells which are superior in light emission properties and reliability to prior art organic EL devices.
In particular, display apparatus in which a plurality of organic EL devices are arranged in a matrix often adopt the time-division drive mode. However, upon driving in the time-division drive mode, the luminance of light emission per pixel becomes relatively lower with the increasing number of time-division. Therefore, when displays having a number of pixels and a relatively large area or high-definition displays are driven in the time-division mode, it becomes difficult to maintain a luminance necessary as a display screen or to carry out multi-gradation control, failing to provide displays of high image quality.
An object of the present invention is to provide an organic EL device which can take advantage of both an organic material and an inorganic material, and has an extended effective life, an improved efficiency, a low drive voltage and a low cost.
Another object of the present invention is to provide an organic EL device which can provide a high luminance of light emission when applied to displays of the time-division drive mode and realize a large screen, high definition display.
Japanese Patent No. 2793383 discloses an organic EL device comprising an insulating metal oxide layer having an energy gap of at least 4.0 eV between a light emitting layer and a cathode. However, this insulating layer is formed between the negative electrode and the light emitting layer and thus differs from the hole injecting and transporting layer which is formed between the hole injecting electrode and the light emitting layer. Also, the materials examined in Examples are only Mgo and BaO. In the prior art section of this patent, it is described to form an SiO2 layer on the ITO electrode as the insulating layer. However, it is pointed out in the patent that when the SiO2 layer is formed by sputtering or similar methods, there occur the malign influences that the organic substance can be melted and crystallized so that its function is deteriorated, and sputtering particles attack the organic substance and alter the interface so that its electron or hole injecting property is lost.
JP-A 37883/1986 which is cited as the prior art reference in the above-referred patent describes an electroluminescent device comprising two electrode layers at least one of which is transparent, two light emitting layers formed between the two electrode layers, and a transparent or translucent electrode formed between the two light emitting layers wherein either one of the two light emitting layers is a monomolecular film of an organic compound or a layer of stacked monomolecular films. Also described therein as a third layer is an insulating layer having the functions of enhancing the insulation of capacitor structure and confining electron transfer within the necessary minimum area to achieve light emission by efficient exchange of electrons. The thickness of this layer is up to 500 angstroms, preferably up to 200 angstroms. It is described in the Example of forming the light emitting layer No.17 that an SiO2 layer of 1,000 angstroms thick is formed on ITO as the insulating layer. However, the electroluminescent device described in this reference is designed for ac driving. Consequently, no reference is made to the function of the insulating layer as a hole injecting layer for positively injecting holes in the dc or pulse drive mode.
JP-A 288069/1996 describes an organic electro-luminescent device comprising an organic layer containing at least a light emitting layer formed between a hole injecting electrode and an electron injecting electrode, wherein an insulating thin film layer is formed between either one of the above-described electrodes and the organic layer. Preferably, a nitride, more preferably aluminum nitride or tantalum nitride is used as the material of which the insulating thin film layer is made. As the material of the insulating thin film layer, GeO, GeO2, SiO, and SiO2 are described. The thickness of this insulating thin film is referred to only in Examples where the thickness is always 50 angstroms. However, it is clearly seen from the above description that the preferred materials of the insulating thin film are nitrides, but not silicon oxide and germanium oxide. This insulating thin film layer plays the role of a buffer layer, while no reference is made to its hole injection and electron blocking functions.
Also, none of these patents describe that the sputtering gas such as argon, krypton, or xenon is introduced into the film being formed, and the content of sputtering gas is described nowhere.
The above objects are achieved by the present invention that is defined below.
(1) An organic electroluminescent device comprising a hole injecting electrode, an electron injecting electrode, at least one organic layer between the electrodes, and an inorganic insulating hole injecting and transporting layer between said hole injecting electrode and said organic layer, wherein
said inorganic insulating hole injecting and transporting layer contains silicon oxide and/or germanium oxide as a main component, the main component being represented by the formula:
(Si1xe2x88x92xGex)Oy
xe2x80x83wherein 0xe2x89xa6xxe2x89xa61 and 1.8xe2x89xa6yxe2x89xa62.5, and further contains 0.01 to 2% by weight of at least one element selected from the group consisting of argon, krypton, xenon, and neon.
(2) The organic electroluminescent device of (1) wherein said inorganic insulating hole injecting and transporting layer has a thickness of 0.1 to 3 nm.